42 Antibody Identification (Matching or Exclusion)

Matching (Direct hit)

The antibody/antigen pattern in the panel must exactly match a pattern of an antigen on the panel cells.

Disadvantage; Only one antibody may be identified. This method is good only when a few cells are reactive.

Exclusion

Exclude antibodies by identifying cells with no reaction (0) between the patient’s plasma and panel cells in any phase (I.S, 37, IAT). The rules for our student labs are as follows:

1. Look at the first cell that has negative results in all phases. The antigens that are present on this cell in double dose are marked with an “X”. If the antigens are present in a single dose the antigen is marked with a “/” not an “X”. Differentiating between homo- (double dose) and hetero-zygous (single dose) antigens, allows antibodies that are showing dosage (only react with homozygous antigen forms) remain as a possibility.
2. When a double dose antigen is too hard to find (as in the case of “K”) exclude based on three negative reactions with single dose cells (3 “/”). “P₁” is excluded by three strikes and marked with three “/” marks because PA antigen strength is variable. Anti-D is a common antibody and can be excluded with any 3 D+ cells (3 “/”).
3. For antigens whose genes are without alleles (f, Lewis, Xga), exclude based on one negative with a single antigen positive cell.
4. Rare antibodies (anti-Cw, -V, -Kpa, -Jsa, -Lua) are not routinely excluded unless an antibody directed against a low frequency Ag is suspected (i.e. the panel cell carrying the Ag is positive and there are no other antibodies or explanations for the positive result).
5. Continue exclusion” crossing off” with each negative cell until the entire panel of cells is complete. At the end of this process, any antigens that have a “/” on them may be “possible antibodies”. Any with no marks are “probable antibodies”. After reviewing the entire panel, perform the same process on the screening cells to provide additional cells in the exclusion process.
6. If necessary, extra “select” panel cells may be required to” rule in or out” an antibody(ies).
7. To identify probable antibodies, the perfect fit or direct hit model is used; which means an antigen shows the exact pattern of positive and negatives. As for the probability to confirm, when confirming probable antibodies there must be 0.05 or less opportunity of being wrong. Students will use the 3+3 rule to confirm (ie 3 antigen positive cells are agglutinated when testing against the patient’s plasma (positive results) and 3 antigen negative cells do NOT agglutinate with the patient’s serum (negative results)). If there are not enough antigen positive or antigen negative cells extra testing using “ select panel cells “must be run using the patient plasma and cells chosen for their specific antigen characteristics to obtain the necessary numbers (3+3) for probability.

Students should consider the following ten questions during antibody identification testing

1. For a single antibody, does the reaction pattern fit only one antibody specificity?
2. Is antibody specificity consistent with the results of the initial antibody screen?
3. Are reaction phases consistent with antibody specificity?
4. If multiple antibodies are present, do they account for the reactions?
5. If the auto-control is negative, are patient red cells negative for the corresponding antigen(s)?
6. Have additional possible antibodies been excluded by selected red cells?
7. Are all variable reaction strengths be explained (dosage, multiple antibodies, reaction phases)?
8. If tested, are antigen-negative donor cells compatible by antiglobulin crossmatch?
9. If the reactions do not fit antibody specificity or if there are results that are improbable, are they explainable?
10. Are results consistent with the conclusion?